1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly, to a system and method for controlling a voltage applied to a common electrode of a liquid crystal display.
2. Description of the Related Art
Liquid crystal displays (LCDs) have become common in a wide variety of applications due to their modest space and power requirements. These characteristics make LCDs very useful in spatially-sensitive and low power applications, such as portable computers, miniature televisions, aircraft, spacecraft, and portable sensors. As LCDs develop further, more applications are likely to incorporate many types of LCD technology.
In general, a typical LCD comprises a layer of liquid crystal sandwiched between two substrates. The LCD is subdivided into pixels, which are addressable via multiple display electrodes formed on one of the substrates. The second substrate, on the other hand, includes a single, relatively large electrode formed on the surface closest to the liquid crystal layer. The electrode serves as a counter electrode, often referred to as the common electrode, to form a capacitance with each of the display electrodes across the liquid crystal layer. When the addressable display electrodes are charged relative to the common electrode using the appropriate signals, the opacity of the liquid crystal changes according to the magnitude of the potential across the liquid crystal. Thus, by providing the appropriate display signals to the various display electrodes, images may be formed on the LCD.
Because the magnitude of the voltage across the liquid crystal layer determines the transmissivity of the pixel, the voltage applied to the common electrode is controlled to ensure the that desired image is formed on the display. Typically, the common electrode is connected to a regulated power supply and a resistive divider to maintain a substantially constant voltage. All of the display electrodes may then be driven with display signals, using the single, constant voltage applied to the common electrode as a reference voltage.
Although controlling the common electrode voltage tends to supply a steady reference voltage for the display signals, a charge differential may be inadvertently formed between the display electrodes and the common electrode and inadvertently change the display. For example, when the same image is maintained on the LCD for an extended period, charge may accumulate across the liquid crystal layer that it may not fully discharge when the image changes. This tends to result in long-term image retention, in which the previous image is still displayed on the LCD even after different data signals for subsequent images are applied. This not only degrades the quality of the image provided by the LCD, but the accumulation of charge may diminish the life of the LCD.
To minimize such undesirable effects, most video systems drive LCDs with alternating current (AC) signals. Specifically, the polarities of the drive signals are periodically reversed, for example for every frame. Thus, the polarity of the potential to be applied between the display electrode and the common electrode in one frame period is opposite to the polarity of the preceding frame period. The voltage applied to the common electrode is set to the midpoint voltage between the peak positive and negative signal voltages provided by the display driver circuit. Consequently, any charge remaining on a display electrode from a signal of one polarity should be negated by the following signal of opposite polarity.
Despite the application of AC signals to the display electrodes, however, a charge differential may nevertheless form across the liquid crystal layer due to variations in the magnitude of the display signals. For example, the power provided by the display signals may occasionally deteriorate under high loading conditions. Consequently, the mean voltage of the display signals tends to drift away from the midpoint between the original peak magnitudes, which is the voltage applied to the common electrode. As a result, a positive or negative charge with respect to the common electrode may accumulate on the display electrodes and degrade the display.
In addition, other characteristics of LCDs may contribute to the retention of voltage across the liquid crystal layer. In particular, display signals are typically supplied to each display electrode using a switching device dedicated to each pixel, commonly a thin film transistor (TFT). The TFTs, however, commonly exhibit a parasitic capacitance between the gate and the source. The magnitude of the parasitic capacitance is usually related to the structure of the TFT, and thus varies according to the individual display's structure. These parasitic capacitances tend to divide the voltage applied to the gate of the TFT, thus changing the effective voltage applied to the gate by the display signal. As a result, the display electrode may not completely charge or discharge in response to a display signal based on the reference potential of the common electrode.
Residual voltage retained on the display electrode may also be attributable to temperature variations of the liquid crystal layer. In particular, the temperature of the liquid crystal layer affects its capacitance, which further affects the characteristics of the capacitive divider formed by the gate and source parasitic capacitance. As a result, variations in the temperature of the liquid crystal layer due to ambient conditions, power supply, or backlighting may contribute to the retention of charge across the electrodes.